The proposed studies will examine aspects of prevention and treatment of venous thromboembolism and will include the following specific projects: 1. Prevention of venous thromboembolism with low dose heparin or external pneumatic calf compression in patients with unstable angina, with measurement in the blood of early indicators of thrombotic events. 2. Investigation of the sex difference in the antithrombotic effects of aspirin, with emphasis on vessel wall prostacyclin production. 3. Assessment of improved methods of pneumatic compression of the legs for prevention of venous thrombosis in neurosurgical patients. 4. Assessment of the efficacy of external pneumatic calf compression for pulmonary embolism in urologic patients. 5. Investigation of the clinical effects of therapy with heparin subfractions with reduced platelet-aggregating side effects.